Process for spray-painting with aqueous paints

ABSTRACT

The invention relates to a process for spray-painting with aqueous paints, in which paints with different colors are processed, recovered paint is recycled, if desired after admixing with newly supplied paint (fresh paint), to the coating operation, and 
     (a) the substrates are selected such that some of the substrates can painted using a mixture of the paints with different colors (&#34;mixed paint&#34;), and 
     (b) the substrates are selected such that the proportion of the paints with different colors in the overall amount of paint processed is from 10 to 90% by weight, and 
     (c) the substrates are selected such that the proportion in which the paints with different colors are processed corresponds to the proportional content of the paints with different colors in the mixed paint, and/or 
     (d) care is taken, by selecting the color of the fresh paint which is admixed, to ensure that the recovered paint has the same color as the mixed paint.

The invention relates to a process for spray-painting with aqueouspaints, in which two or more aqueous paints with different hues orcolors are processed in a single coating unit and the paint which hasnot gone onto the substrate to be painted (overspray) is washed out ofthe exhaust air from the coating unit and washed off the parts of thecoating unit which come into contact with overspray, using water, andthe water in which overspray has accumulated is concentrated and isrecycled as recovered aqueous paint, if desired after admixing withnewly supplied paint (fresh paint), to the coating operation.

In spray-painting with aqueous paints the paint which has not gone ontothe substrate to be painted (overspray) is washed out of the exhaust airfrom the coating unit and washed off the parts of the coating unit whichcome into contact with overspray, using water. In this process water inwhich overspray has accumulated is obtained, and has to be disposed of.For this purpose the overspray is precipitated from the water in whichoverspray has accumulated. This results in a paint sludge, which canonly be reprocessed to give other products to a limited extent and whichin many cases has to be disposed of as waste.

Alternative proposals for recovering the overspray from water in whichoverspray has accumulated include processes in which the water in whichoverspray has accumulated is concentrated until the solids concentrationof the aqueous paint originally employed is reached again. The recoveredaqueous paint obtained in this way can be recycled, directly or afteradmixing with newly supplied paint (fresh paint), to the coatingoperation (cf. for example Journal fur Oberflaachentechnik (JOT), 1992,issue 10, pp. 32 to 38).

This process is well suited when one paint in one color is processedover relatively long periods of time. However, in practice--especiallywhen finishing car bodies--a frequent change in the colors of the paintsemployed is necessary. The recovered paint then has a mixed color whichvaries, in some cases continuously, depending on the color and quantityof the processed paints. Hitherto, paints of mixed color recovered fromunits in which paints with different colors are processed have beenemployed for painting those regions of the substrates to be coated whichare generally out of sight. This solution to the problem isunsatisfactory because, in general--if at all [sic]--it is not possibleto use all of the quantity of recovered paint obtained in a coating unitin this way. The remaining quantity of recovered paint must then,disadvantageously, either be processed in other coating units, asdescribed in Journal fur Oberflachentechnik (JOT), 1993, issue 3, onpages 27, 30 and 31 depigmented by microfiltration or even disposed ofas unusable waste. In the Journal fur Oberflachentechnik (JOT), 1993,issue 3, pages 26 to 32 it is proposed, moreover, to extend coatingunits in which the color of the paints processed undergoes frequentchanges by the installation of additional working tanks, tanks forretained material and pumps, in order to enable the separate processingof the overspray from the different paints. This solution to theproblem, however, has the disadvantages that it is associated withadditional costs, and, moreover, that it requires the coating unit to becleaned before each change of color.

The object of the present invention consists in the provision of aprocess for spray-painting, of the type described above, in which thedisadvantages of the prior art are eliminated or reduced.

It has been possible, surprisingly, to achieve this object by a processfor spray-painting, of the type described above, which is characterizedin that

(a) the nature and amount of the substrates to be painted in the coatingunit are selected such that some of the substrates to be painted in thecoating unit can be painted using a mixture of the aqueous paints withdifferent hues or colors ("mixed paint") which are to be processed inthe coating unit, and

(b) in that [sic] the nature and amount of the substrates to be paintedin the coating unit are selected such that the total amount of aqueouspaints of different hues or colors processed in the coating unit, as aproportion of the overall amount of paint processed in the coating unit(=sum of the total amount of aqueous paints with different colorsprocessed in the coating unit and the total amount of recovered aqueouspaint which may contain fresh paint and is processed in the coatingunit).is from 10 to 90, preferably from 30 to 70, particularlypreferably from 40 to 60% by weight, and

(c) in that [sic] the nature and amount of the substrates to be paintedin the coating unit are selected such that the proportion in which theaqueous paints of different hues or colors are processed correspondsapproximately to the proportional content of the aqueous paints withdifferent colors in the mixed paint described in a), and/or

(d) care is taken, by selecting the color of the fresh paint which isadmixed if desired, to ensure that the recovered aqueous paint hasessentially the same color as the mixed paint described in (a).

The process according to the invention makes it possible to utilize allof the recovered aqueous paint in the same coating unit, even in coatingunits in which a frequent change is necessary in the colors of thepaints processed, without extending the coating unit and withsubstantially lower expenditure for cleaning the coating unit.

Processes for spray-painting with aqueous paints, in which two or moreaqueous paints with different colors are processed in a single coatingunit and the paint which has not gone onto the substrate to be painted(overspray) is washed out of the exhaust air from the coating unit andwashed off the parts of the coating unit which come into contact withoverspray, using water, and the water in which overspray has accumulatedis concentrated and is recycled as recovered aqueous paint, if desiredafter admixing with newly supplied paint (fresh paint), to the coatingoperation, are known and are described in, for example, Journal furOberflachentechnik (JOT), 1992, issue 10, page 32 to 38 and in Journalfur Oberflachentechnik (JOT), 1993, issue 3, page 26 to 32. With regardto further information on the prior art, in particular with regard tothe washing-out or washing-off of the overspray, to the concentration ofthe water in which overspray has accumulated and to the general designof coating units which are suitable for the processes under discussion,reference is made to EP-A-307 047, EP-A-245 863, EP-A-137 877, EP-A-127685, EP-A-318 827, EP-A-271 015, EP-A-217 212, DE-A-38 00980 andCH-4615/89-4.

The water in which overspray has accumulated can be concentrated in avariety of ways, for example by mechanical deposition using, forexample, centrifuges or decanters, with the aid of membrane processes,especially with the aid of ultrafiltration, with the aid of thermalprocesses (e.g. vacuum evaporation) or with the use of electrophoreticdeposition, or by a combination of one or more of the methods listed.The concentration of the water in which overspray has accumulated ispreferably carried out with the aid of ultrafiltration. Apparatussuitable for concentrating the water in which overspray is accumulatedis commercially available from, for example, Eisenman, HolzgerlingenFRG.

Those features of the process according to the invention which areessential to the invention will be explained exemplarily using theexample of the finishing of car bodies with aqueous fillers of differentcolors.

The conventional finishing of car bodies comprises a first step in whichan electrodeposition coat is applied, over which a filler coatis-applied after the baking operation. The filler coat is then baked or,while still unbaked, is coated over with a one-coat or multi-coat, inparticular a two-coat, topcoat finish. Finally the topcoat is baked, thefiller coat also being baked in cases where the topcoat has been appliedto the unbaked filler coat.

The principal functions of the filler coat are to compensate forunevenness and defects in the electro-deposition coat and to improve thestone-chip resistance of the overall finish.

There is often a desire, however, to provide the car bodies withtopcoats which, because of the nature and quantity of the color pigmentsemployed, are not able completely to hide the substrate at the coatthicknesses conventionally employed. In such cases the fillers employedthemselves have a color which is such that the non-hiding topcoatexhibits the desired color. Fillers with colors adapted to the color ofthe topcoat are also employed in many cases so that, in the case ofdamage caused by stone chips, the damaged areas of the finish are not soobvious, as is the case with filler coats whose color is very differentfrom that of the topcoat. Often, because of the changing colors of thetopcoats, the color of the fillers employed must also vary accordingly.The nature and quantity of the oar bodies to be finished with thedifferent-colored topcoats is regulated by the demand from the market.To allow the process according to the invention to be carried out, in afirst step it must be ascertained which different colors are absolutelynecessary for the fillers to be employed. The colors of the fillersdepend--as outlined above--on the colors of the desired topcoats. Assoon as it is established how many fillers with different colors arerequired, which colors these different-colored fillers are to have andin what quantities the respective fillers are used, in a second step amixture of the fillers with different colors is prepared, which mixturecontains the fillers with different colors in the proportion in whichthe different-colored fillers are used. The resulting "mixed filler" or"mixed paint" is then tested as to the extent to which it can beemployed as filler, on the basis of its color, for the maximum amount oftopcoats to be applied. By varying the proportions in which the fillerswith different colors are mixed, the color of the "mixed filler" or"mixed paint" can be varied until the quantity of topcoats to beprocessed, and for which the "mixed filler" or "mixed paint" can beemployed on the basis of its color, is as large as possible, and, at thesame time, until the proportional content of the fillers with differentcolors in the "mixed filler" or "mixed paint" is as close as possible tothe proportion in which the fillers with different colors are used.

To carry out the process according to the invention, it is alsonecessary for the nature and amount of the substrates to be painted (thecar bodies to be finished in the example described above), which are tobe painted in the coating unit under discussion, to be selected suchthat the total amount of aqueous paints of different colors processed inthe coating unit, as a proportion of the overall amount of paintprocessed in the coating unit (=sum of the total amount of aqueouspaints with different colors processed in the coating unit and the totalamount of recovered aqueous paint which may contain fresh paint and isprocessed in the coating unit) is from 10 to 90, preferably from 30 to70, particularly preferably from 40 to 60% by weight.

In order to enable the recycling of as much as possible of the recoveredaqueous paint, care is to be taken in accordance with the invention toensure that the recovered aqueous paint, which may contain admixed freshpaint, has essentially the same color as the "mixed paint"("mixedfiller" in the example) described above. "Essentially" is intended todenote that it is often possible in practice to tolerate more or lessslight deviations in color, but that it is preferred for the color ofthe recovered aqueous paint to match the color of the "mixed paint".

The color of the recovered aqueous paint can be influenced, by selectingthe proportion in which the aqueous paints with different colors areprocessed and/or by selecting the color of the fresh paint which isadmixed if desired, such that it essentially matches the color of themixed paint described in (a). The proportion in which the aqueous paintswith different colors are processed is selected by way of acorresponding selection of the substrates to be painted in the coatingunit. The closer the proportion in which the aqueous paints withdifferent colors are processed is to the proportional content of theaqueous paints with different colors in the mixed paint described in(a), the less the need for correction by adding appropriately coloredfresh paints. The formulation "approximately" should be understood, incontext, like the formulation "essentially" explained above. Thepossibilities for adapting the color of the recovered aqueous paint tothe color of the "mixed paint", by admixing appropriately colored freshpaints, become lesser the higher the total amount of aqueous paints withdifferent colors processed in the coating unit as a proportion of theoverall amount of paint processed in the coating unit. This means that,as the proportion of the aqueous paints with different colors which areprocessed in the coating unit in the overall amount of paint processedin the coating unit becomes higher, the adaptation of the color of therecovered paint to the color of the mixed paint must be effected to anincreasing extent by way of the proportion in which the aqueous paintswith different colors are processed, which means that the proportion inwhich the aqueous paints with different colors are processed, as theproportion of aqueous paints with different colors which are processedin the coating unit in the overall amount of paint processed in thecoating unit becomes higher, is permitted to deviate less and less fromthe proportional content of the aqueous paints with different colors inthe "mixed paint".

The process according to the invention is preferably employed for thefinishing of car bodies with aqueous fillers which have differentcolors. It can, however, be applied to all spray-painting processesusing aqueous paints, in which two or more aqueous paints with differentcolors are processed in a single coating unit and the paint which hasnot gone onto the substrate to be painted (overspray) is washed out ofthe exhaust air from the coating unit and washed off the parts of thecoating unit which come into contact with overspray, using water, andthe water in which over-spray has accumulated is concentrated and isrecycled as recovered aqueous paint, if desired after admixing withnewly supplied paint (fresh paint), to the coating operation.

We claim:
 1. Process for spray painting one or more substrates in acoating unit with aqueous paints, wherein two or more aqueous paintswith different hues or colors are processed in a single coating unit,comprising applying the paints to the substrates, and recovering theoverspray of the paints, wherein the overspray is a mixture of thepaints of different hues or colors, optionally, adding fresh paint tothe recovered paint, selecting one or more substrates, where the desiredcoating color for the substrate is the color of the recovered mixedpaint, and reusing the recovered paint in the spray painting process tocoat the selected substrate, wherein overspray is recovered by(i)washing with water, the overspray paint from the exhaust air of thecoating unit and from the parts of the coating unit which come intocontact with overspray; and ii) concentrating the water in whichoverspray has accumulated to form recovered aqueous paint which is amixture of paints of different colors;wherein the amount of aqueouspaints of different hues or colors processed in the coating unit is from10 to 90% by weight, based on the sum total of paint processed in thecoating unit, the sum total including aqueous paints of different huesor colors initially processed, recovered paint and fresh paint which isoptionally added to the recovered paint, and the proportion of theaqueous paints having different hues or colors in the recovered paintcorresponds approximately to the proportional content of the aqueouspaints having different hues or colors in the paints initially appliedin the coating unit.
 2. The process of claim 1, further comprising thestep of adding fresh paint to the recovered aqueous paint mixture. 3.Process according to claim 2, further comprising adding fresh paint tothe recovered paint to ensure that the recovered aqueous paint has thesame proportion of colors as the proportion of colors applied initiallyin the coating unit.
 4. Process according to claim 1, wherein thesubstrates painted in the coating unit are car bodies which have beencoated with an electrodeposition coat.
 5. Process according to claim 1,wherein the aqueous paint is an aqueous filler paint for the finishingof car bodies.
 6. Process for spray painting one or more substrates in acoating unit with aqueous paints, wherein two or more aqueous paintswith different hues or colors are processed in a single coating unit,comprising applying the paints to the substrates, and recovering theoverspray of the paints, wherein the overspray is a mixture of thepaints of different hues or colors, adding fresh paint to the recoveredpaint, selecting one or more substrates, where the desired coating colorfor the substrate is the color of the recovered mixed paint, and reusingthe recovered paint in the spray painting process to coat the selectedsubstrate, wherein overspray is recovered by(i) washing with water, theoverspray paint from the exhaust air of the coating unit and from theparts of the coating unit which come into contact with overspray; andii) concentrating the water in which overspray has accumulated to formrecovered aqueous paint which is a mixture of paints of different huesor colors;wherein the amount of aqueous paints of different hues orcolors processed in the coating unit is from 10 to 90% by weight, basedon the sum total of paint processed in the coating unit, the sum totalincluding aqueous paint of different hues or colors initially processed,recovered paint and fresh paint which is added to the recovered paint,and the fresh paint is added to the recovered paint to match theproportion of the aqueous paints having different hues or colors in therecovered paint to the proportion of the aqueous paints having differenthues or colors in the paints initially applied in the coating unit. 7.The process of claim 6 wherein the coating applied to the substrate is afiller coating.